98 dodge ram 1500 has crappy heat , when fan is on full speed its almost like it blowing cold air, turn the fan down and the cab heats up ? when you hammer on the throttle the air flow changes to full defrost weather it has the heat on or the air on in the summer ?? I have had the heater core flushed , changed the thermostat , no change still crappy heat any ideas thanks mike

There is a one way check valve in the vacuum system, that is why is it is ramdomly changing from defrost to floor etc.
Be certain the rad if full to the top, then (when cold) let it idle for a while to release any air bubbles.
Another common problem is the blend doors need repair, check you tube.

the heater controls are vacuum operated and they have a reservoir under the cowl. these crack. you can pull the dash control and apply vacuum to each actuator from the plug in connector. this will tell you if you have a bad actuator.

I have a 99 ram and had terrible heat. Also didn't have any a/c due to the evaporator leaking. So I decided to pull the dash, put in a new evap. core, replaced the heater core just because I had it apart and also had to replace the blend door actuator. It wasn't closing all the way and letting cold air enter with the hot air thus cooling the heated air down. I thought the 98's were cable operated still but I may be wrong. The actuator motor was 110.00 on mine. Now I can't even turn the blower on unless it's under freezing outside it works so well.

There is also a vacuum reservoir under the cowl on the passenger side that you will need to replace. I bypassed mine and it works much better.

'98-02 are hybrid systems. All controls are vacuum EXCEPT the blend door, which is heat. In the passenger foot well on the bottom of the case you'll see an electric actuator.
You can pull the two screws and drop it down if you move the carpet. Check the spline to see if it's stripped and the spline door drive in the case as well. You can manually move the door to full heat with the motor off. This will tell you if the door is broke. Most often I see the case leak into the actuator and short the motor. Leave the actuator plugged in and turn the knob from hot to cold and see if the motor spline moves.

BTW, Dodge has an inline vacuum canister/check valve for the A/C vacuum feed that will solve your problem with airflow switching.